Hallman 1 ABSTRACT the Epizonal Mount Aetna Caldera in Central
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ABSTRACT The epizonal Mount Aetna caldera in central Colorado consists of the Mount Aetna pluton and the associated large-volume Badger Creek Tuff. New CA-TIMS U/Pb zircon ages are consistent with field relations that demonstrate that the Badger Creek Tuff was erupted during the incremental assembly of the ~34.9-34.3 Ma Mount Aetna pluton. The youngest dated portion of the pluton intrudes the ~34.5 Ma Badger Creek Tuff and yields a weighted mean 206Pb/238U age of 34.28 ± 0.06 Ma. This sample also contains 34.9 Ma antecrysts that date the onset of pluton emplacement, but none that are similar in age to the ignimbrite. Similarly, the Badger Creek Tuff has no antecrysts that date the emplacement of the pluton. New major and trace element data are consistent with older studies, and suggest that the Badger Creek Tuff is not the fractionated equivalent of the Mount Aetna pluton. The disparity in age between these units also precludes this possibility. We interpret the eruption of the Badger Creek Tuff to be the result of a sudden and temporary spike in magma flux against the background low-flux assembly of the Mount Aetna pluton. INTRODUCTION Two endmember hypotheses have been proposed to explain the spatial and temporal connections between intrusive rocks and associated large-volume (>400 km3) ignimbrites. The first suggests that large ignimbrites are the erupted fraction of even larger magma chambers that underwent differentiation and fractional crystallization in the upper crust (Bowen, 1928; Bachmann and Bergantz, 2004; Hildreth, 2004), and that the unerupted portion of the chamber is preserved as plutonic rocks. The second hypothesizes that most plutons are incrementally assembled during periods of low magma flux, and that large volume ignimbrites Hallman 1 (“supereruptions”) are erupted during periods of high magma flux, perhaps leaving very little in the plutonic rock record (Glazner et al., 2004; Tappa et al., 2011; Zimmerer and McIntosh, 2012; Mills and Coleman, 2013). Understanding the connections between intrusive rocks and large-volume ignimbrites is directly tied to deciphering the mechanisms that govern crustal growth and the processes responsible for supereruptions. Eruptions as small as 100-200 km3 have been tied to temporary changes in climate that directly affect the ability of humans to produce crops (Stothers, 2004), and the 2000 km3 75 ka eruption of Toba (Matthews et al., 2012) is thought to have reduced the human population to a few thousand (Ellis and Mark, 2013). Understanding the processes and rates at which large eruptible bodies are produced is important for anticipating and mitigating hazards associated with large eruptions. To better understand the relationships between plutonic rocks and broadly contemporaneous volcanic rocks, it is important to recognize a general intrusive and eruptive sequence. Field relations can be verified with geochronology, which can be used to derive detailed intrusive and eruptive histories for magmatic centers. Because of its unambiguous field relations, the Mount Aetna caldera complex, which exposes the Mount Aetna pluton and the associated Badger Creek Tuff, is an ideal location to test the connections between plutonic and large-volume volcanic magmatism. Geochronologic data (Mills and Coleman, 2013) suggest that the pluton (34.95 ± 0.04 Ma) is somewhat older than the tuff (34.47 ± 0.05 Ma); however, the existing map for the caldera and local field relations indicate that the Mount Aetna pluton is resurgent into the tuff (Shannon, 1988; Mills and Coleman, 2013). These apparently contradictory observations might suggest that the Mount Aetna pluton was incrementally assembled across the period of time marked by eruption of the Badger Creek Tuff. Alternatively, Hallman 2 because the field relations unambiguously demonstrate that at least part of the pluton is younger than the tuff, perhaps the U-Pb zircon data do not accurately date intrusion of the Mount Aetna pluton. Evaluating this possibility is critical for the interpretation of zircon ages from any pluton. The traditional caldera model (Bowen, 1928; Smith and Bailey, 1968; Bachmann and Bergantz, 2004; Hildreth, 2004) predicts essentially synchronous ages between the Badger Creek Tuff and the Mount Aetna pluton, which would have been emplaced over a short period largely preceding eruption of the tuff. Alternatively, the flux-dependent hypothesis (Glazner et al., 2004; Tappa et al., 2011; Zimmerer and McIntosh, 2012; Mills and Coleman, 2013) predicts that the Mount Aetna pluton should have been incrementally assembled over perhaps several hundred thousand years that span a period of ignimbrite quiescence. In order to evaluate these two endmember hypotheses, we provide new high-precision CA-TIMS U-Pb zircon geochronology for the porphyritic Mount Aetna Quartz Monzonite where it intrudes the Badger Creek Tuff (MPRM-DSC), a fine-grained dike that cuts the porphyritic Mount Aetna pluton (MA13-01), and new fractions of the fine-grained facies of the Mount Aetna pluton (MPRM-20) previously dated by Mills and Coleman (2013). GEOLOGIC SETTING The Mount Aetna caldera complex (Figure 1) is exposed in the southern portion of the late Eocene Mount Princeton batholith in the central Colorado volcanic field. The Mount Aetna caldera is structurally within the Sawatch Range block, which is adjacent to the Arkansas River valley along the main fault of the Rio Grande rift system (Mills and Coleman, 2013). Magmatism at the Mount Aetna caldera began following Laramide shortening and prior to the onset of Rio Grande extension, which helped to expose the system through Neogene faulting and Hallman 3 erosion (Epis and Chapin, 1975; Shannon, 1988). The calc-alkaline chemistry and spatially linear distribution of magmatism within the central Colorado volcanic field led most authors (Coney and Reynolds, 1977; Lawton and McMillan, 1999; Zimmerer and McIntosh, 2012) to attribute volcanism to the decoupling of the Farallon plate from the North American plate following flat- slab subduction, which brought volcanism ~1200 km inland from the trench; however, others (Mutschler et al., 1987) dispute the subduction hypothesis and suggest that the magmatism reflects the early onset of Rio Grande rift tectonics. Hallman 4 Figure 1. Simplified geologic map of the Mount Aetna caldera including sample locations (modified from Shannon, 1988; Mills and Coleman, 2013). Hallman 5 The Mount Aetna pluton is a quartz monzonite that intrudes the ~36.0-35.3 Ma Mount Princeton batholith (Mills and Coleman, 2013). An earlier study (Mills and Coleman, 2013) reports a weighted mean CA-TIMS 206Pb/238U zircon age of 34.95 ± 0.04 Ma for the porphyritic Mount Aetna Quartz Monzonite and 34.47 ± 0.05 Ma for the Badger Creek Tuff. A ring dike, a tuff dike, and a fine-grained phase of the Mount Aetna pluton (MPRM-20) were found to be the same age as the Badger Creek Tuff within uncertainty (Mills and Coleman, 2013). A portion of the porphyritic Mount Aetna pluton (MPRM-DSC) intrudes the Badger Creek Tuff, which is inconsistent with published geochronology (Mills and Coleman, 2013), motivating this study. METHODS U-Pb Geochronology Samples were prepared for zircon separation with a jaw crusher and split into fractions for geochronology and geochemical analysis. The fraction for geochronology was further processed with a disc mill, and mineral separations were carried out using a water table, heavy liquids, and a magnetic separator. Zircon fractions were hand-picked using a binocular microscope. Select grains were thermally annealed for 48 hours at 850°C and chemically abraded for 8-16 hours in a pressure-dissolution vessel containing 6 M HCl at 180°C in order to remove damaged zones that were open to Pb diffusion (Mattinson, 2005). Fractions were picked and spiked with a 205Pb-233U-236U tracer and then dissolved in 29 M HF in a pressure solution vessel for approximately 90 hours at 220°C before conversion to a chloride salt. The separation of U and Pb was accomplished with HCl anion exchange column chromatography. Isotopic ratios (Table 1) were measured with a thermal ionization VG Sector 54 mass spectrometer employing a Daly detector. Silica gel was used to load U and Pb onto single Re Hallman 6 filaments, and U was run as an oxide. Corrections for initial Th/U disequilibrium were made using the method of Mattinson (1973), and whole rock trace element data from Mills and Coleman (2013). Elemental chemistry Samples were prepared with a steel jaw crusher and manually split into representative aliquots for elemental chemistry. The aliquot for chemistry was powdered with a ceramic shatterbox and ignited at 950°C to expel volatiles (Lechler and Desilets, 1987). Major element analysis was performed on lithium metaborate/tetraborate flux discs. Trace element analysis was performed on pressed powder discs that were prepared using paraffin and a 25 ton press. Elemental analysis was conducted by XRF in the Department of Geological Sciences (Tables 2 and 3). RESULTS U/Pb geochronology New fractions of a fine-grained intrusion (MPRM-20) previously dated by Mills and Coleman (2013) are generally consistent with the older data, but also include new, younger fractions ranging in age from ~34.7 to 34.3 Ma that yield a weighted mean 206Pb/238U age of 34.32 ± 0.03 Ma. A newly identified fine-grained dike that cuts the Mount Aetna Quartz Monzonite (MA13-01) could be traced for approximately 200 m on a NNE-SSW bearing. It yields an age of 34.37 ± 0.04 Ma. Fractions are consistent in age except for an ~35.6 Ma antecryst that is similar in age to the Mount Princeton batholith (Mills and Coleman, 2013). It includes no fractions that are similar in age to the early period of emplacement of the porphyritic Mount Aetna pluton at ~34.95 Ma (Mills and Coleman, 2013). A sample of the Mount Aetna Hallman 7 pluton where it intrudes the Badger Creek Tuff (MPRM-DSC) yields a weighted mean 206Pb/238U age of 34.28 ± 0.06 Ma.